KR20130075034A - Method for producing thick steel plate having excellent welded zone toughness and ductility and weld structure using the same mathod - Google Patents

Abstract

PURPOSE: A manufacturing method of thick steel plate having the excellent toughness and ductility of welded part and a welded structure using the same are provided to have excellent toughness and ductility of SAW (Submerged Arc Welding) welded part, in which the component of base material is diluted during welding process, by controlling the rolling and cooling condition. CONSTITUTION: A manufacturing method of thick steel plate having the excellent toughness and ductility of welded part comprises the following steps of: heating a steel material at 1000 to 1250°C; completing the finishing rolling of heated steel material over the ferrite transformation initial temperature (Ar3); and cooling the rolled steel material at temperature over the Ar3. The steel material includes carbon of 0.06 to 0.14 wt%, manganese of 1.0 to 1.6 wt%, silicone of 0.1 to 0.5 wt%, niobium of 0.005 to 0.06 wt%, and phosphorus of 0.008 to 0.03 wt%. The steel material cp is satisfied with 0.38 to 0.64, and remaining part consists of iron (Fe) and the other unavoidable impurities. [Reference numerals] (AA) Welded part ductility (J); (BB) Ductility; (CC) Scope of the request for Cp; (DD) Elongation percentage; (EE) Elongation percentage (%)

Description

Manufacturing method of thick steel plate with excellent weld toughness and ductility, and welded structure using the same

The present invention relates to a thick steel sheet having excellent weld toughness and excellent ductility of steel, and a method of manufacturing the same.

Steel for welding structure is welded to produce a structure, wherein the properties of the weld is deteriorated by the welding. Therefore, it is necessary to reinforce the physical properties of the weld, and in particular, it is very important to improve the toughness of the weld.

In order to solve the above problems, Patent Literature 1 proposes a technique for preventing the deterioration of the structure of the weld heat affected zone by dispersing the composite precipitates with Ti nitride and Nb nitride by controlling the content of Ti, Nb, N, and the like. .

In addition, as a similar technique, Patent Literature 2 proposes a technique for improving the toughness of a welded portion by using Ti and B to prevent the deterioration of the structure of the welded heat affected zone in a wide heat input range.

All of the above-described prior arts have the effect of preventing tissue deterioration of the welded part to improve the toughness of the welded part. There is a problem that the effect is limited. That is, the above technique is a technique of suppressing tissue deterioration by using precipitates such as carbonitrides. However, since the precipitates are dissolved together with the weld metal in the molten paper, the effect cannot be exerted at the site of resolidification after re-dissolution.

In order to improve the toughness of the part that is dissolved and solidified again during the welding process (called 'welding metal'), a method of adding a large amount of expensive elements such as Ni and Mo to the welding material may be used. However, this method can not only increase the cost of the excessive welding material, but also almost no effect in the SAW welding method in which the base material is re-dissolved in a large amount of 70% or more and diluted with the welding metal to form a weld metal.

Republic of Korea Patent Publication No. 2007-0091971 Republic of Korea Patent Publication No. 2008-0091716

One aspect of the present invention is to solve the above-described problems, a method of manufacturing a thick steel plate for welded structure having excellent toughness and excellent ductility and welded structure using the same SAW welded portion of the base material is diluted to a large amount of welding metal during welding It aims to provide.

The present invention is by weight, C: 0.06 ~ 0.14%, Mn: 1.0 ~ 1.6%, Si: 0.1 ~ 0.5%, Sol. Al: 0.005 to 0.06%, Nb: 0.005 to 0.05%, P: 0.008 to 0.03%, Cp represented by the following relation 1 satisfies 0.38 to 0.64, and the balance is made of steel made of Fe and other unavoidable impurities Heating at 1000 to 1250 ° C .; Completing the finish rolling of the heated steel at a ferrite transformation start temperature (Ar3) or more; And relates to a method for producing a welded steel plate excellent in weldability toughness and ductility comprising the step of cooling the rolled steel at a temperature of Ar3 or more.

<Relation 1>

Cp = C + Nb x 5.3 + P x 7.1 + Si / 9.4 + Mn / 7.8

Here, C, Nb, P, Si, and Mn each represent weight% of the corresponding element.

In addition, the present invention is by weight, C: 0.06 ~ 0.14%, Mn: 1.0 ~ 1.6%, Si: 0.1 ~ 0.5%, Sol. Al: 0.005 to 0.06%, Nb: 0.005 to 0.05%, P: 0.008 to 0.03%, the component index (Cp) represented by the above relation 1 satisfies 0.38 to 0.64, the structure of the weld portion is less than 10% It relates to a welded structure with excellent weld toughness and ductility, characterized in that the ferrite (Ferrite) and more than 90% bainite (Bainite) and acicular ferrite (Acicular ferrite).

According to the present invention, it is possible to manufacture a thick steel sheet for welded structures having excellent toughness and excellent ductility in a SAW weld where the components of the base material are diluted in the welding process by controlling the steel alloy components, and controlling the rolling and cooling conditions.

1 is a view showing a change in impact toughness and elongation of the SAW welding portion according to the component index (Cp).

Hereinafter, a description will be given in detail of embodiments of a method for manufacturing a welded steel plate and a welded structure using the same according to the present invention, but the present invention is not limited to the following examples. Therefore, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Hereinafter, the present invention will be described in detail.

According to the present invention, a method for manufacturing a welded steel sheet having excellent weld toughness and ductility is in weight%, C: 0.06 to 0.14%, Mn: 1.0 to 1.6%, Si: 0.1 to 0.5%, and Sol. Al: 0.005 to 0.06%, Nb: 0.005 to 0.05%, P: 0.008 to 0.03%, the component index (Cp) represented by the following relation 1 satisfies 0.38 to 0.64, the balance is Fe and other unavoidable impurities The steel material which consists of this is heated at 1000-1250 degreeC, finish rolling is completed above ferrite transformation start temperature (Ar3), and cooling starts at the temperature of Ar3 or more.

<Relation 1>

Cp = C + Nb x 5.3 + P x 7.1 + Si / 9.4 + Mn / 7.8

Here, C, Nb, P, Si, and Mn each represent weight% of the corresponding element.

At this time, the composition of the steel may further contain Ti: 0.005 ~ 0.03% by weight and N: 0.001 ~ 0.008% by weight, the content of S (sulfur), which is inevitably included in the steel is S: 0.005% by weight It is preferable to restrict to the following.

At this time, it is preferable to start the cooling at a cooling rate of 1 to 20 ℃ / sec at the start of the cooling, and to stop the cooling in the temperature range of 500 to 700 ℃.

Hereinafter, the reason for limiting the components in the thick steel sheet for welded structure of the present invention will be described in detail.

At this time, the content of the component element means all weight%.

C: 0.06-0.14%

Carbon (C) is preferably added in an amount of 0.06% or more as an effective element for causing solid solution strengthening and improving the hardenability of steel to improve strength. However, when excessively added, the fraction of pearlite is increased to lower the ductility of the base material, and the upper limit is limited to 0.14% because the toughness of the heat affected zone (HAZ) is extremely degraded.

Mn: 1.0-1.6%

Manganese (Mn) is an important constituent of the present invention because it is diluted with the weld metal during SAW welding to improve the structure of the weld metal and, as a result, to improve the toughness of the SAW weld. In order to produce such an effect, it is necessary to add 1.0% or more. However, when excessively added, the toughness of the weld heat affected zone can be reduced, so the upper limit thereof is limited to 1.6%.

Si: 0.1-0.5%

Since silicon (Si) serves to deoxidize molten steel by assisting aluminum, it is useful to obtain a deoxidation effect, so it is necessary to add 0.1% or more. However, when excessively added, the island martensite formed in the weld heat affected zone is not decomposed, so there is a high risk of brittle fracture. Therefore, the upper limit is limited to 0.5%.

Sol. Al: 0.005-0.06%

Aluminum (Al) is a major deoxidizer in steel and needs to be added at least 0.005%. However, if the content exceeds 0.06%, the upper limit is limited to 0.06% because not only the deoxidation effect is saturated but also the oxide can be increased to reduce the toughness.

Nb: 0.005-0.05%

Niobium (Nb) is diluted with the weld metal during SAW welding to improve the structure of the weld metal and, consequently, to enhance the toughness of the welded part, and is also effective for improving the base metal structure. One of the elements In order to obtain the above effects, it is necessary to add 0.005% or more, but when added excessively, the effect of improving the structure of the weld is saturated, the ductility can be greatly reduced, and the toughness of the weld heat affected zone can be reduced. Therefore, the upper limit is limited to 0.05%.

P: 0.008 ~ 0.03%

Phosphorus (P) is generally treated as an impurity in the steel and the content is as low as possible. However, in the present invention, since it is diluted with the weld metal during SAW welding together with Mn, Nb and the like to improve the structure of the weld metal to increase the toughness of the weld, it is one of the most important members together with Nb. To achieve this effect, it is necessary to add more than 0.008%, but if excessively added, the effect of improving the structure of the weld zone is saturated, and the toughness of the weld heat affected zone and the base material can be greatly reduced by grain boundary segregation. The upper limit is limited to 0.03%.

More preferably, it is added in 0.012 to 0.03% range.

In order to further improve the ductility and toughness of the weld heat affected zone, Ti and N satisfying the following conditions may be further added to the steel of the present invention.

Ti: 0.005-0.03%

Titanium (Ti) combines with oxygen or nitrogen to form various types of precipitates to refine the structure to improve the ductility of the steel, and also serves to prevent tissue degradation of the weld heat affected zone. It is necessary to add 0.005% or more in order to obtain the above effect. However, when added in excess of 0.03%, the effect is saturated, so it is preferable to limit it to 0.03% or less.

N: 0.001-0.008%

Nitrogen (N) is a component contained in the steelmaking process, and if the content is 0.001% or less, it is necessary to add 0.001% or more because the effective role of Ti is impossible. However, when added excessively, it is preferable to limit the upper limit to 0.008% because it causes age hardening and greatly reduces toughness.

In addition, the steel of the present invention may include impurities that cannot be completely removed in the steelmaking process, sulfur (S) of the impurities is preferably limited to the following conditions to further improve the properties of the steel.

S: 0.005% or less

Sulfur (S) is an element that causes the red heat brittleness of the steel, the lower the advantage, but the upper limit is limited to 0.005% in consideration of the load of the steelmaking process.

In addition to the composition, the steel according to the present invention preferably has a component index (Cp) represented by the following relational formula (1) is 0.38 to 0.64.

The component index (Cp) is an important component of the present invention, and when Cp is 0.38 or less, it is difficult to improve the toughness of the weld metal part because the effect of improving the structure of the weld metal part cannot be exhibited. On the other hand, in the case of 0.64 or more, the structure improvement effect of the weld metal part is almost saturated, but rather, the toughness and ductility of the weld heat affected part are deteriorated.

<Relation 1>

Cp = C + Nb x 5.3 + P x 7.1 + Si / 9.4 + Mn / 7.8

Here, C, Nb, P, Si, and Mn each represent weight% of the corresponding element.

The relationship between the impact toughness of the SAW weld metal part and the elongation change of the base material according to the component index (Cp) according to Equation 1 is shown in FIG. 1.

As shown in FIG. 1, the Cp should be 0.38 or more to secure the impact toughness of the welded portion 60J or more as the welded structural steel, and the Cp should be 0.64 or less to secure the elongation of 20% or more appropriate as the structural steel. .

Hereinafter, the manufacturing method of the thick steel plate for welded structures which satisfy | fills the above-mentioned steel component is demonstrated in detail.

The following manufacturing method shows a preferred example in which the thick steel sheet for welding structure of the present invention can be manufactured, but is not limited thereto.

It is preferable to roll in the following manner in order to combine the weld part toughness, the ductility and the strength of a base material as steel materials of the conditions mentioned above.

Rolling and cooling conditions

Reheating Temperature: 1000 ~ 1250 ℃

The process of reheating a slab or ingot in a furnace for hot pressing is required. At this time, when the reheating temperature is too low, less than 1000 ℃ there is a problem that the load is largely applied during rolling, the alloy component is not sufficiently dissolved. On the other hand, when the reheating temperature is too high, the upper limit is limited to 1250 ° C. because the grains grow excessively in the heating furnace to reduce toughness.

Finish rolling finished temperature: above Ar3 temperature

If the rolling is carried out below the Ar3 temperature, the transformed ferrite is processed, which can greatly deteriorate the ductility of the base material. Therefore, finish rolling must be completed at a temperature of Ar3 or higher.

Cooling start temperature: Above Ar3 temperature

When the hot rolled steel sheet is cooled as described above, when the cooling start temperature is lower than the Ar3 temperature, ferrite transformation is performed at a slow cooling rate during air cooling, thereby forming coarse ferrite, which may greatly reduce strength or reduce ductility. . Therefore, it is necessary to start cooling at a temperature of Ar3 or higher. However, the restriction on the cooling start temperature is not applied to all cases, and the steel sheet is thick so that the cooling rate described below is not satisfied due to natural cooling in air. Apply.

Cooling rate: 1 ~ 20 ℃ / sec

If the cooling rate is very slow at less than 1 ° C./sec after finishing rolling, the lower limit is 1 ° C./sec because not only it is difficult to secure the predetermined strength but also the ferrite is formed too large to reduce the ductility. Limited to On the other hand, if the cooling rate is too fast, it is preferable to limit the upper limit to 20 ° C / sec because low-temperature tissues such as bainite or martensite may be formed to reduce toughness or greatly reduce ductility.

Cooling end temperature: 500 ~ 700 ℃

When the cooling rate is set to 1 to 20 ° C / sec as described above, it is necessary to stop the cooling in the temperature range of 500 to 700 ° C. When cooling of steel is stopped at 700 ℃ or higher, it is difficult to secure a predetermined strength due to insufficient ferrite refining effect.On the other hand, when cooling to a low temperature of 500 ℃ or lower, low temperature structure or phase such as bainite Martensite is produced, which can greatly degrade the ductility and toughness of the steel.

However, if the steel is thin or small in size and a cooling rate of 1 ° C./sec is achieved by natural air cooling (air cooling), it is preferable to cool in air after rolling regardless of the above cooling rate and cooling end temperature. Do.

SAW welding is applied to the welded structure steel (base material) that satisfies the above component system and manufacturing conditions, the microstructure of the weld is less than 10% ferrite (Ferrite) and 90% or more bainite or acicular ferrite (Acicular) ferrite).

In this case, the component index (Cp) represented by the following relation 1 should satisfy 0.38 to 0.64. If the Cp value is less than 0.38, 10% or more of ferrite is formed in the welded part after welding, which causes a problem that the toughness of the welded part is lowered.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following examples are only examples for describing the present invention in more detail, and do not limit the scope of the present invention.

< Example >

In order to observe the effect of the composition and the manufacturing method of the steel sheet according to the present invention, the rolling was carried out in the production conditions shown in Table 2 for the steel materials of the composition and conditions shown in Table 1.

Table 1 below describes 15 kinds of steels (inventive steels) and nine kinds of steels (comparative steels) outside the composition range of the present invention, which satisfy the composition of the steel, which is the subject of the present invention. The rolling method for steel of composition is described.

Inventive materials 1 to 4 in Table 2 shows a rolling method meeting the conditions of the present invention with respect to the invention steel 1, Comparative materials 1 to 6 show a rolling method outside the conditions of the present invention. In addition, Inventive Materials 5 to 18 show a rolling method that satisfies the conditions of the present invention for steel materials (Inventive Steels 2 to 15 in Table 1) that satisfy the range of the component composition of the present invention, and Comparative Materials 7 to 15 are For steels (compare steels 1 to 9 in Table 1) that do not satisfy the component composition range of the present invention, a rolling method meeting the conditions of the present invention is shown.

number C Si Mn P S Al Nb Ti N Cp Ar3 Inventive Steel 1 0.10 0.29 1.31 0.017 0.003 0.027 0.021 - 0.0041 0.57 780 Invention river 2 0.06 0.34 1.20 0.015 0.004 0.007 0.030 - 0.0038 0.55 801 Invention steel 3 0.14 0.16 1.07 0.013 0.002 0.036 0.011 - 0.0013 0.47 787 Inventive Steel 4 0.11 0.10 1.24 0.017 0.003 0.031 0.029 - 0.0037 0.59 783 Invention steel 5 0.08 0.50 1.14 0.011 0.002 0.031 0.010 - 0.0029 0.44 800 Invention steel 6 0.12 0.38 1.01 0.013 0.003 0.023 0.031 - 0.0042 0.58 799 Invention steel 7 0.07 0.23 1.60 0.012 0.002 0.025 0.017 - 0.0033 0.50 766 Inventive Steel 8 0.11 0.34 1.31 0.008 0.003 0.047 0.023 - 0.0057 0.51 778 Invention river 9 0.07 0.17 1.05 0.03 0.002 0.034 0.008 - 0.0042 0.55 810 Invented Steel 10 0.12 0.34 1.43 0.021 0.002 0.037 0.005 - 0.0027 0.56 764 Invention steel 11 0.08 0.13 1.11 0.011 0.002 0.017 0.050 - 0.0056 0.60 802 Invention steel 12 0.07 0.14 1.10 0.010 0.002 0.018 0.013 - 0.0043 0.39 806 Invention steel 13 0.12 0.38 1.52 0.018 0.002 0.054 0.021 - 0.0036 0.64 757 Comparative River 1 0.15 0.45 1.55 0.017 0.003 0.034 0.013 - 0.0036 0.63 746 Comparative River 2 0.13 0.60 1.70 0.015 0.002 0.026 0.013 - 0.0049 0.62 740 Comparative Steel 3 0.05 0.21 0.90 0.019 0.003 0.037 0.012 - 0.0034 0.43 829 Comparative Steel 4 0.10 0.24 1.10 0.004 0.003 0.033 0.014 - 0.0033 0.38 797 Comparative Steel 5 0.13 0.24 1.00 0.032 0.003 0.033 0.009 - 0.0039 0.63 796 Comparative Steel 6 0.11 0.23 1.22 0.011 0.003 0.025 0.003 - 0.0043 0.41 784 Comparative Steel 7 0.10 0.30 1.01 0.01 0.004 0.043 0.052 - 0.0036 0.63 805 Comparative Steel 8 0.065 0.13 1.04 0.009 0.002 0.019 0.007 - 0.0043 0.33 813 Comparative Steel 9 0.13 0.48 1.46 0.026 0.002 0.054 0.016 - 0.0036 0.70 759 Invented Steel 14 0.09 0.31 1.25 0.013 0.003 0.027 0.016 0.005 0.0016 0.49 788 Invented Steel 15 0.09 0.29 1.26 0.012 0.002 0.018 0.015 0.03 0.0080 0.47 787

Here, the content of each element refers to the weight percent, Cp is the result calculated from the weight percent of C, Mn, Si, Nb, P by the formula 1, Ar3 is the temperature at which the ferrite transformation starts in austenite it means.

division number Finish rolling
Temperature (℃) Cooling
Start temperature (℃) Cooling rate
(℃ / sec) Cooling
Stop temperature (℃) Ar3
(℃) Inventory 1 Inventive Steel 1 806 788 5.6 560 780 Inventory 2 Inventive Steel 1 930 912 7.3 620 780 Comparison 1 Inventive Steel 1 724 713 5.4 548 780 Comparative material 2 Inventive Steel 1 782 747 3.2 670 780 Invention 3 Inventive Steel 1 809 790 1.0 500 780 Invention 4 Inventive Steel 1 808 788 20.0 700 780 Comparative material 3 Inventive Steel 1 807 787 0.5 560 780 Comparison 4 Inventive Steel 1 810 793 28.0 520 780 Comparative material 5 Inventive Steel 1 809 789 9.3 411 780 Comparative material 6 Inventive Steel 1 928 911 5.3 785 780 Invention Article 5 Invention river 2 841 825 9.3 618 801 Inventions 6 Invention steel 3 866 842 8.2 598 787 Invention 7 Inventive Steel 4 832 814 9.2 623 783 Invention 8 Invention steel 5 864 841 11.2 637 800 Invention 9 Invention steel 6 862 840 10.5 645 799 Inventions 10 Invention steel 7 842 814 9.9 625 766 Invention invention 11 Inventive Steel 8 854 824 11.3 615 778 Invention 12 Invention river 9 861 832 11.9 611 810 Invention invention 13 Invented Steel 10 843 812 8.8 586 764 Invention Article 14 Invention steel 11 874 847 9.3 622 802 Invention material 15 Invention steel 12 877 848 9.7 636 806 Invention material 16 Invention steel 13 848 815 10.7 613 757 Comparison 7 Comparative River 1 857 822 8.3 563 746 COMPARISON 8 Comparative River 2 854 825 14.8 525 740 Comparative material 9 Comparative Steel 3 893 862 3.6 677 829 Comparative material 10 Comparative Steel 4 862 830 8.9 545 797 Comparative material 11 Comparative Steel 5 927 898 9.2 535 796 Comparative material 12 Comparative Steel 6 871 838 9.7 585 784 Comparative material 13 Comparative Steel 7 870 832 11.5 554 805 Comparative material 14 Comparative Steel 8 877 843 10.7 564 813 Comparative material 15 Comparative Steel 9 847 823 8.6 634 759 Invention 17 Invented Steel 14 848 816 10.8 602 788 Invention 18 Invented Steel 15 853 819 10.9 615 787

Table 3 shows the physical property test results of the steel rolled and cooled under the conditions of Table 2 to the slabs formed under the conditions of Table 1.

division number Yield strength
(MPa) The tensile strength
(MPa) Elongation
(%) SAW weld
Impact Toughness (J) HAZ
Impact Toughness (J) Inventory 1 Inventive Steel 1 334 482 23 132 164 Inventory 2 Inventive Steel 1 319 474 24 126 154 Comparison 1 Inventive Steel 1 456 523 16 123 137 Comparative material 2 Inventive Steel 1 273 362 22 127 111 Invention 3 Inventive Steel 1 272 439 23 126 127 Invention 4 Inventive Steel 1 348 495 22 122 113 Comparative material 3 Inventive Steel 1 233 365 24 122 106 Comparison 4 Inventive Steel 1 425 562 17 126 114 Comparative material 5 Inventive Steel 1 441 574 16 126 121 Comparative material 6 Inventive Steel 1 226 373 23 124 112 Invention Article 5 Invention river 2 327 497 24 126 143 Inventions 6 Invention steel 3 322 492 27 106 92 Invention 7 Inventive Steel 4 331 505 23 138 119 Invention 8 Invention steel 5 312 452 27 86 102 Invention 9 Invention steel 6 309 426 24 137 118 Inventions 10 Invention steel 7 346 464 25 127 132 Invention invention 11 Inventive Steel 8 336 489 25 121 123 Invention 12 Invention river 9 297 504 24 134 157 Invention invention 13 Invented Steel 10 326 523 24 136 135 Invention Article 14 Invention steel 11 415 536 21 138 107 Invention material 15 Invention steel 12 264 418 29 65 164 Invention material 16 Invention steel 13 421 548 20 137 87 Comparison 7 Comparative River 1 412 553 20 125 54 COMPARISON 8 Comparative River 2 408 537 19 128 43 Comparative material 9 Comparative Steel 3 264 387 29 52 124 Comparative material 10 Comparative Steel 4 341 475 28 54 136 Comparative material 11 Comparative Steel 5 402 514 21 127 48 Comparative material 12 Comparative Steel 6 297 442 27 47 112 Comparative material 13 Comparative Steel 7 442 541 20 132 57 Comparative material 14 Comparative Steel 8 248 406 29 10 121 Comparative material 15 Comparative Steel 9 453 571 15 143 85 Invention 17 Invented Steel 14 330 489 26 112 127 Invention 18 Invented Steel 15 327 475 27 106 131

In Table 3, SAW welds and HAZ impact toughness were measured using a Charpy V-notch impact test at -20 ° C.

As shown in Table 3, the steel produced by the rolling and cooling method of the invention steel satisfying the component range according to the present invention is 20% or more elongation, 60J or more welded and HAZ toughness, yield strength of 235 MPa or more and 400 Having a tensile strength of MPa or more, the results were very good for use as a welded structural steel.

However, although the steel (invention steel 1) satisfying the component range according to the present invention, Comparative materials 1 and 2 whose finish rolling temperature or cooling start temperature is out of the range of the present invention are 20% or less in ductility, or the tensile strength of the base metal. It can be seen that is less than 400 MPa is not suitable for use as a welded structural steel. In particular, the comparative material 1 is a result of the ferrite is severely hardened due to the rolling is made in the ideal zone is not broken in the weld metal and not in the base metal, the comparative material 2 is a result of the cooling is started in the ideal zone, a large amount of soft large ferrite formed Caused by.

Comparative materials 3 to 6 are also steels satisfying the component range according to the present invention (invention steel 1), but the cooling conditions are outside the scope of the present invention, and the strength or elongation is remarkably poor, which makes them unsuitable for use in steel for welding structures. Do. Particularly, the comparative material 3 and the comparative material 6 are very slow cooling rate and very high cooling end temperature, respectively, and are the result of the formation of ferrite too large or the fraction of the ferrite to decrease the strength. Comparative material 5 is a case where a high cooling rate and a low cooling end temperature are applied, respectively, because low-temperature structures such as bainite are formed by the above conditions, which adversely affects elongation.

Comparative materials 7 to 15 are obtained by rolling and cooling steel (comparative steels 1 to 9) outside the component range of the present invention under the conditions according to the present invention, wherein at least one of elongation, toughness of a weld, or toughness of a welded HAZ is reduced. It is not suitable for use as steel for welding structure.

Particularly, Comparative Material 7 is obtained by rolling and cooling Comparative Steel 1, in which the content of carbon (0.15% by weight) exceeds the upper limit according to the present invention, and has a high carbon content, such as phase martensite and bainite in HAZ. This large amount was formed to lower the HAZ impact toughness.

Comparative material 8 is a case in which the content of Si and Mn (0.60, 1.70 wt%, respectively) in the components of Comparative Steel 2 added in excess of the upper limit of the present invention is rolled and cooled according to the present invention. Similar to the addition, the pearlite fraction increased, the hardenability of the steel increased too much, the elongation was low, and the impact toughness was also low in the HAZ.

Comparative material 9 is a case in which the comparative steel 3, in which the contents of C and Mn (0.05 and 0.90 wt%, respectively) is added below the lower limit of the present invention, is rolled and cooled according to the present invention. There is no shortage for use as steel, but the impact toughness of SAW weld was very low, 52J. This is attributable to the lack of alloying elements of the base material, thereby reducing the hardenability of the SAW weld portion having a high dilution rate, resulting in coarse ferrite and grain boundary ferrite.

Comparative material 10 can be seen that when the content of P (0.004% by weight) is added to less than the lower limit of the present invention when comparative steel 4 is rolled and cooled according to the present invention, the impact toughness of SAW welds is significantly lower. This is due to the decrease in the content of P in the SAW welding portion, thereby reducing the hardenability, and consequently, coarse ferrite and grain boundary ferrite were formed.

Comparative material 11 is a case in which the comparative steel 5, in which the P content (0.032% by weight) is added in excess of the upper limit of the present invention, is rolled and cooled according to the present invention. You can see the value. This is the result of P causing embrittlement in HAZ.

Comparative material 12 is a case in which the comparative steel 6, in which the Nb content (0.003% by weight) is added below the lower limit of the present invention, is rolled and cooled according to the present invention. . This is because there was a lack of Nb content in the SAW welding portion, and the hardenability was reduced, and the structure micronization effect of Nb was hardly exhibited.

Comparative material 13 is a case in which the comparative steel 7 in which the Nb content (0.052% by weight) is added in excess of the upper limit of the present invention is rolled and cooled according to the present invention. You can see the value. This is the result of embrittlement by Nb precipitation in HAZ.

Comparative material 14 is a case in which the comparative steel 8, which has a value of Cp lower than the lower limit of the present invention, is rolled and cooled according to the present invention. This is due to the excessively low value of Cp, an important feature of the present invention, that the structure of the SAW weld metal having a large dilution rate is not improved and coarse ferrite and grain boundary ferrite are formed.

Comparative material 15 is a case in which the comparative steel 9, whose Cp value is excessively higher than the upper limit of the present invention, is rolled and cooled according to the present invention, and the strength of the base material and the impact toughness of the SAW weld are excellent, and the HAZ impact toughness is also good. It can be seen that the elongation of the weld metal is very low. This is because the hard tissue is formed by excessive increase of Cp and the elongation is lowered.

As described above, when the steel prepared by the present invention is manufactured by the manufacturing method according to the present invention, it can be confirmed that there is an excellent effect on the welded structural steel, in particular, the welded structural steel to which SAW welding with a high dilution rate of the base material is applied. have.

As a result, the present invention provides a method of optimally controlling not only the composition of the alloy element but also the rolling and cooling conditions in order to solve not only the ductility of the weld but also the difficulty of ensuring excellent impact toughness of both the weld metal and the HAZ. According to the present invention as well as the impact toughness of the HAZ and SAW welds, which are important characteristics of the welded structural steel, it is possible to effectively produce a welded structural steel excellent in ductility and strength of the base material.

Claims (7)

By weight%, C: 0.06-0.14%, Mn: 1.0-1.6%, Si: 0.1-0.5%, Sol. Al: 0.005 to 0.06%, Nb: 0.005 to 0.05%, P: 0.008 to 0.03%, Cp represented by the following relation 1 satisfies 0.38 to 0.64, and the balance is made of steel made of Fe and other unavoidable impurities Heating at 1000 to 1250 ° C .;
Completing the finish rolling of the heated steel at a ferrite transformation start temperature (Ar3) or more; And
Cooling the rolled steel at a temperature of Ar3 or higher;
Method of manufacturing a thick steel sheet for welded structure having excellent weld toughness and ductility comprising a.
<Relationship 1>
Cp = C + Nb x 5.3 + P x 7.1 + Si / 9.4 + Mn / 7.8
Here, C, Nb, P, Si, and Mn each represent weight% of the corresponding element.
The method of claim 1, wherein the steel further comprises 0.005 to 0.03 wt% of Ti and 0.001 to 0.008 wt% of N.
The method of claim 1 or 2, wherein the content of S (sulfur), which is an impurity inevitably included in the steel, S: 0.005% by weight or less, characterized in that the weld steel thick steel sheet for excellent weld structure toughness and ductility Manufacturing method.
2. The welded steel plate having excellent weld toughness and ductility according to claim 1, wherein the cooling step is performed at a cooling rate of 1 to 20 deg. C / sec and stops cooling at a temperature range of 500 to 700 deg. Manufacturing method.
By weight%, C: 0.06-0.14%, Mn: 1.0-1.6%, Si: 0.1-0.5%, Sol. Al: 0.005 to 0.06%, Nb: 0.005 to 0.05%, P: 0.008 to 0.03%, and the component index (Cp) represented by the following relation 1 satisfies 0.38 to 0.64, and the welded structure is less than 10% The weld structure excellent in toughness and ductility of the weld, characterized in that the ferrite (Ferrite) and more than 90% bainite (Bainite) or acicular ferrite (Acicular ferrite).
<Relationship 1>
Component Index (Cp) = C + Nb × 5.3 + P × 7.1 + Si / 9.4 + Mn / 7.8
Here, C, Nb, P, Si, and Mn each represent weight% of the corresponding element.
The weld structure of claim 5, wherein the thick steel sheet further contains 0.005 to 0.03 wt% of Ti and 0.001 to 0.008 wt% of N.
The weld structure having excellent weld toughness and ductility according to claim 5 or 6, wherein the content of S (sulfur), which is inevitably contained in the steel sheet, is limited to S: 0.005 wt% or less.

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